Verres chalcogénures pour mémoires électriques : Caractérisation par spectroscopie Raman et microscopie en champ proche

Chalcogenide materials which can be used in the development of electrical memory were studied in the course of this work. The aim was to improve our knowledge of the materials and of the mechanisms that could explain the electrical switching. The study was not focused on the crystalline/amorphous phase change phenomena involved, for example, in the stoechiometric material Ge2Sb2Te5 (GST) at the basis of the development of PC-RAM (Phase Change Random Access Memory). We rather focused on the electrical switching phenomena which can be used in the development of R-RAM memories (Resistive Random Access Memory). Two kinds of materials were studied: Ag-Ge-Se compounds selected to develop "programmable metallization cell" and Ge2Sb2+xTe5 compounds containing an excess of antimony as compared to the GST phase change material. First, a structural and electrical investigation of the active material Agx(GeySe1-y)100-x as a bulk glass and as a thin film was carried out, mainly on the basis of characterization by near field microscopy and Raman spectroscopy. Then, the electrical switching in thin films was investigated with a new method: the conductive atomic force microscopy (C-AFM). The same technique allowed investigating the electrical switching phenomena in thin films prepared by sputtering Ge2Sb2+xTe5 (x = 0.25, 0.5, 1) targets.Ce travail s'inscrit dans l'étude de matériaux chalcogénures susceptibles d'être utilisés dans le développement de mémoires électriques. Notre but était d'apporter un éclairage sur les matériaux et sur les mécanismes susceptibles d'expliquer les commutations électriques en leur sein. Nous ne nous sommes pas intéressés aux phénomènes de changement de phase cristallisé/amorphe intervenant, par exemple, dans le matériau stoechiométrique Ge2Sb2Te5 et à la base du développement des mémoires PC-RAM (Phase Change Random Access Memory). Nous nous sommes plutôt concentrés sur les phénomènes de commutation électrique pouvant être utilisés dans le développement de mémoires R-RAM (Resistive-Random Access Memory). Deux types de matériaux ont été étudiés : les matériaux Ag-Ge-Se sélectionnés pour le développement de " cellules à métallisation programmable " et les matériaux e2Sb2+xTe5 contenant un excès d'antimoine. Dans le premier cas, nous avons d'abord réalisé une étude fondamentale du matériau actif Agx(GeySe1-y)100-x sous forme de verre massif et de film mince. La microscopie en champ proche et la spectroscopie Raman ont été les techniques d'analyse de choix au cours de cette étude. Puis nous avons procédé à une étude de la commutation électrique au sein des films minces par une méthode originale : la microscopie à force atomique conductrice. Cette même technique nous a permis d'étudier les phénomènes de commutation électrique dans des films minces préparés par pulvérisation cathodique de cibles Ge2Sb2+xTe5 contenant un excès d'antimoine (x = 0.25, 0.5, 1)

Verres chalcogénures pour mémoires électriques (caractérisation par spectroscopie Raman et microscopie en champ proche)

Ce travail s'inscrit dans l'étude de matériaux chalcogénures susceptibles d'être utilisés dans le développement de mémoires électriques. Notre but était d'apporter un éclairage sur les matériaux et sur les mécanismes susceptibles d'expliquer les commutations électriques en leur sein. Nous ne nous sommes pas intéressés aux phénomènes de changement de phase cristallisé/amorphe intervenant, par exemple, dans le matériau stœchiométrique Ge2Sb2Te5 et à la base du développement des mémoires PC-RAM (Phase Change Random Access Memory). Nous nous sommes plutôt concentrés sur les phénomènes de commutation électrique pouvant être utilisés dans le développement de mémoires R-RAM (Resistive-Random Access Memory). Deux types de matériaux ont été étudiés : les matériaux Ag-Ge-Se sélectionnés pour le développement de cellules à métallisation programmable et les matériaux Ge2Sb2+xTe5 contenant un excès d'antimoine. Dans le premier cas, nous avons d'abord réalisé une étude fondamentale du matériau actif Agx(GeySe1-y)100-x sous forme de verre massif et de film mince. La microscopie en champ proche et la spectroscopie Raman ont été les techniques d'analyse de choix au cours de cette étude. Puis nous avons procédé à une étude de la commutation électrique au sein des films minces par une méthode originale : la microscopie à force atomique conductrice. Cette même technique nous a permis d'étudier les phénomènes de commutation électrique dans des films minces préparés par pulvérisation cathodique de cibles Ge2Sb2+xTe5 contenant un excès d'antimoine (x = 0.25, 0.5, 1)MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Morphology and structural studies of Ag photo-diffused GeySe1y thin films prepared by RF-sputtering

International audienceThin films of Ag photo-doped-GeySe1-y films were prepared by RF co-sputtering technique. A systematic study of the relation existing between the host layer composition and the saturation rate in silver was carried out. Morphology and composition studies before and after chemical etchings were performed by scanning electron microscopy and electron probe micro-analyses, respectively. Raman spectroscopy studies showed the presence of corner-sharing and edge-sharing Ge(Se1/2)4 tetrahedra in all thin films. The vibration mode corresponding to Sen-chains is observed for the Ge-poor host layer and on the contrary, Ge-rich thin films exhibited some tendency to form homopolar Ge–Ge bonds as a part of ethanelike Ge2Se6 units. These investigations revealed the amount of Ag that could be incorporated in the host layer. Such an amount strongly depends on the relative ratio Ge/Se in the GeySe1-y thin film. For the Gepoor host layer, an incorporation of Ag (54 at.%) was observed but also a drastic increase in the film heterogeneity. On the other hand, the host layers with higher Ge content showed homogeneous surfaces and a saturation level of 32–41 at.% Ag

High resolution electrical characterisation of Ag-conducting heterogeneous chalcogenide glasses

International audienceElectric force microscopy (EFM) and conductive atomic force microcopy (C-AFM) are introduced to perform nanoscale electrical characterization of phase separated Agx(Ge0.25Se0.75)100−x glasses. Changes in the relative permittivity are found for both phases when the silver content is changed. Furthermore, the sensitivity of the C-AFM technique revealed current variations of a few pico-amperes in the Ag-rich phase for the different glass compositions. This result confirms that the increase in conductivity of the Ag–Ge–Se samples in the region of high ionic conduction (xN8–10 at.%) arises from an increase in conductivity of the Ag-rich phase and not from an increase in amount of Ag-rich phase with a fixed composition and conductivity

Bipolar resistance switching in chalcogenide materials

International audienceThe paper reports on an investigation of two chalcogenide films that both show bipolar resistance switching, i.e., Ag/Ge0.25Se0.75 and Ge2Sb3Te5. Changes occurring in the chalcogenide films during switching were analyzed by conductive atomic force microscopy (C-AFM). All the findings in the first Ag/Ge0.25Se0.75 family are in agreement with the proposal of a migration of Ag from the electrode throughout the film, creating random conductive paths when a bias is applied and their rupture when the bias is reversed. Reversible bipolar resistance switching with bias in the range of few volts was clearly demonstrated in Ge2Sb3Te5 films, even though its nature is not so well understood. Clear enough is the fact that a primary and irreversible contraction of the film along with an increase in its conductivity occurred when a bias was applied to the film. It was related to a crystallization of the film. Further write/erase cycles induced resistance switching but no change in the contraction/expansion of the films. Our findings corroborated a mechanism where an amorphous Sb-rich phase would exist in between Ge2Sb2Te5 crystallites. This would create conductive paths when a bias is applied, paths that would break when the polarity is reversed

Nanostructured ionic conductors: A study of Vycor®7930–LiI composites

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NANOSTRUCTURED IONIC CONDUCTORS: INVESTIGATION OF SiO2-MI (M = Li, Ag) COMPOSITES

Synthesis and characterization of porous Vycor (R) 7930-MI (M = Li, Ag) composites are discussed. Two types of composites were prepared: sintered composites obtained by classical sintering (CS) and composites obtained by electro-crystallisation (EC). In the case of Lil composites all increase of the conductivity of two orders of magnitude as compared to the conductivity Of pure Lil was observed. In Such a case good coating of insulating particles and filling of the pores was observed. In the case of Agl the main important finding is the presence of an hysteresis phenomenon in the conductivity versus temperature Curve at a temperature close to the transition alpha bet

Ag-Ge-Se glasses: a vibrational spectroscopy study

International audienceAgx(Ge0.25Se0.75)100 x glasses with x varying from 0 to 25 have been shown to exhibit a conductivity threshold around x ~8-10. In this work, the structural changes induced by introduction of Ag in Ge25Se75 glass have been investigated using Raman and infra-red spectroscopies. In ambient conditions, changes are observed in position, width and intensity for vibrations assigned to Ge-Se bonds, showing that the tetrahedral network is relaxed and gains flexibility as Ag is introduced. High pressure experiments on two glasses containing 5 and 25 at.% Ag confirm that Ag-rich (25%) glasses clearly exhibit higher compressibility than Ag-poor (5%) glasses

Anomalous Conductivity Behavior of AgI-Vycor7930 Nanocomposites

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Ionic conductivity in nanoporous composites SiO2/AgI

5 pagesInternational audienceIn this work, porous silicate glasses were mixed with an ionic conductor AgI to obtain SiO2/AgI composites. The porous glasses were either commercial Vycor glass (VPG) from Corning or synthesised from phase separated SiO2–B2O3–Na2O glasses. Depending upon the elaboration process, glasses with different pore sizes were obtained (4, 20 and 40 nm). Composites comprising different amounts of AgI were prepared by heating above the melting point of AgI. SEM characterisation of the obtained composites indicated that heat treatment induced an increase in the pore sizes. The conductivity of the composites was measured by impedance spectroscopy. The maximum conductivity, twice as large as that of pure AgI, was obtained for the 50AgI/50VPG composite, i.e. that comprising the glasses with the smallest pores